The present invention relates to a differential pressure sensor using the piezoresistivity effect, and more particularly to a differential pressure sensor that has a diaphragm on which a diffused resistor is formed as a pressure-sensitive element and to which a fluid pressure subject to measurement is introduced.
One conventional semiconductor differential sensor includes two absolute pressure sensor units each having a diaphragm, to which fluid pressures are applied for measurement, and a device for determining the difference between the absolute pressure outputs of the two sensor units. Another conventional semiconductor differential sensor is the type having a diaphragm to opposite surfaces of which fluid pressures subject to measurement are applied to produce an output representative of the difference between the two applied fluid pressures.
In FIG. 1 of the accompanying drawings is shown an absolute pressure sensor unit of the former type, generally designated by 10. This sensor unit includes a diaphragm block 12 having a diaphragm 14 and made of a silicon chip on the front surface 16 of which diffused resistors 18 are formed while the opposite or back surface 20 of the diaphragm is subject to the fluid pressure to be measured. The diaphragm block 12 is bonded to an alumina base 22 by means of a bonding layer 24 and covered by a cap 26 secured to the base 22 such that the front surface 16 of the diaphragm 14 on which the diffused layers 18 are formed is positioned within a vacuum chamber 28 formed by the base 22 and the cap 26. The base 22 has a hole 30 formed therethrough communicating with an inlet pipe 32 connected to the outside surface of the base. When a fluid pressure is introduced through the inlet pipe 32 and the hole 30 to back 20 of the diaphragm 14, the introduced pressure is converted to a corresponding electrical signal according to the stress-resistance characteristics of the diffused resistors 18. Accordingly, if two fluid pressures are introduced so as to arrive at the respective diaphragms 14 of two such sensor units, these sensor units will produce corresponding outputs from which the pressure difference can be obtained. However, use of two sensor units in this way leads to making the sensor necessarily bulky.
FIG. 2 shows one example of the latter differential sensor which has a single diaphragm 14 and which is structurally the same as the sensor of FIG. 1 except that it has an inlet pipe 34 connected to a through hole in the cap 26 through which another fluid pressure is introduced to the front 16 of the diaphragm 14. When two fluid pressures P.sub.1 `and P.sub.2 are introduced through the inlet pipes 32 and 34 to opposite surfaces of the diaphragm 16, the diffused resistors 18 on the diaphragm 16 produce an electrical signal corresponding to the difference between the introduced fluid pressures P1 and P2. This pressure sensor 10 is smaller than the sensor of FIG. 1, but since the diffused resistors 18 are exposed to the fluid of one of the pressures subject to measurement, they are liable to deteriorate due to moisture and corrosive gases which may be contained in the fluid.